The primary goal of this research is to determine whether the pattern of peripheral innervation in the vertebrate embryo is regulated by intrinsic factors related to lineage. Specifically, the experiments are designed to test the hypothesis that clonal and compartmental lineage relationships may be involved in guiding the first developing sensory and motor axons (pioneer axons) to their targets. Peripheral tissue and the central nervous system of the frog are parcellated into compartments, each one of which is derived from a specific ancestral cell group in the 512-cell stage blastula. The proposed research will investigate whether pioneer axons grow along specific pathways to specific targets by recognizing nonneuronal cells belonging to the same compartment and/or clone. Lineage analysis will be performed by marking clones in the frog blastula. Individual blastomeres will be intracellularly injected with horseradish peroxidase. This marker does not spread to adjacent cells, is inherited by all progeny of the injected cell and is not toxic to normal development. Members of the marked clone will be identified by histochemical procedures that render the reaction product visual at both the light and electron microscopic levels. At the ultrastructural level, I will determine whether pioneer axons use clonally related peripheral cells as substrates during pathway formation, and whether pioneer axons preferentially synapse on clonally related targets cells. In addition, peripheral compartments will be ablated, or transplanted to ectopic locations, in order to determine whether pionner axons will grow ectopically to associate with compartmentally related tissues. Finally, the sensory pioneer neurons will be ablated to determine whether their presence is required for the observed confinement of motor pioneer axons to compartmentally related tissues in normal embryos. This research will enhance our understanding of peripheral axonal guidance, since the clone marking technique to be used will allow a lineage analysis of neuronal connectivity for the first time in a vertebrate embryo.